In certain medical treatment procedures, a type of endoprosthesis device known as a stent is placed or implanted within a blood vessel for treating various problems such as stenonses, strictures, or aneurysms in the blood vessel. These devices are implanted within the vascular system to reinforce collapsing, partially occluded, weakened or abnormally dilated sections of the blood vessel. Stents may also be implanted in the ureter, urethra, bile duct, or any body vessel which has been narrowed, weakened or in any of the other ways which requires reinforcement.
A common approach for implanting stents in peripheral or coronary arteries is to first open the constricted region of the vessel via a percutaneous transluminally inserted angioplasty balloon catheter. The uninflated balloon at the tip of the catheter is advanced into the narrowed portion of the vessel lumen. The balloon is inflated so as to push the stenotic plaque outward, thereby enlarging the luminal diameter. Thereafter another catheter containing the stent is advanced to the region just enlarged by the balloon catheter and the stent is deployed. The catheter is withdrawn leaving the stent within the vessel.
The concept of implanting transluminally placed coil spring stents within an artery is not new. In one experiment in 1969, six stents were implanted in arteries of dogs. Three stents were stainless steel covered with silicone rubber and the other three stents were bare stainless steel. All three silicone coated stents occluded within 24 hours while two of the three bare stents remained open for thirty months. The stents were deployed using a pusher catheter having the same outer diameter as the stent.
In 1983, thermally expandable stents were reported, in which an alloy wire was shaped at thigh temperature into a stent configuration. Later it was straightened at room temperature into a configuration suitable for transluminal placement. Once placed within the vessel the stent was exposed to elevated temperatures to cause the alloy to return to its initial coil configuration. Canine studies of these stents, using the alloy nitinol, an alloy of nickel and titanium, demonstrated restenosis and intimal thickening 8 weeks following implant.
In 1984, self-expanding stents were described in which a device was introduced percutaneously after torsion reduction and was deployed by applying a reverse torsion in-vivo. This type of device proved to be complex and limited by a small expansion ration. Another self-expanding stent used stainless steel wire in a zig zag configuration which resulted in incomplete vascular contact and only partial healing of the device. Yet another mechanical self-expanding stent was reported where a woven multifiliment stainless steel stent was deployed by a catheter with a constricting outer sleeve. Once in place, the outer sleeve was removed allowing self-expansion of the spring stent against the vessel wall.
Thrombosis occurred in these early prototypes, especially when the vessel tapered, and at branch points and at low expansion ratios. Canine aortic implantation resulted in multiple areas of vessel-to-stent adhesion at 3 weeks following implant. The stent exhibited minimal thrombogenicity.
Balloon expandable stents were first reported as being constructed of woven stainless steel wire where the cross points were silver soldered to resist radial collapse. The stent was deployed unexpanded over a balloon catheter, and once in position the stent was expanded by the outward force of the balloon. 8 of 11 stents implanted remained open for 1 to 8 weeks. It has been observed that the amount of intimal hyperplasia to be inversely proportional to the initial vessel lumen diameter. In another version, silver soldering cross points were replaced by the use of a stainless steel tube with rows of offset slots which became diamond shaped spaces. Although neointimal hyperplasia was observed, all stents remained open in rabbit aortas for 6 months.
Placement of a stent in a blood vessel is described in Lindemann et al U.S. Pat. No. 4,878,906 where a combination of sheath covered sleeve and a balloon catheter are used to locate and place the prosthesis. No recognition is given to the problems just discussed herein.
A prosthesis system using an expandable insert is shown in Garza et al U.S. Pat. No. 4,665,918, which is typical of those devices which are implanted without any express concern for the biocompatibility of the device being inserted. One can expect many of the foregoing problems and concerns to be evidenced by this device.
One device which is shown in U.S. Pat. No. 4,768,507 to Fischell et at describes a coil spring stent on which an application of a carbon coating or a carbon coated polytetrafluoroethylene has been applied on the surface of the coil spring. Fischell et al teaches that the thrombogenic potential of the device is reduced, through a passive methodology, but does nothing to address the biological response to the implant as a foreign body. Moreover, no suggestion is made of a way to inhibit neointimal hyperplasia, which inevitably follows balloon catheter induced injury to arterial vessels.
Yasuda U.S. Pat. No. 4,994,298 employs plasma polymerization to form a thin flexible coating on stents, teaching that improved biocompatibility, such as non-thrombogenicity and tissue or blood compatibility may be improved. Again this process is a passive methodology as previously described.
There are essentially two types of stents which have been employed in the prior art. Spring like stents have been inserted using a sheath or restraining element to keep the spring from expanding until It is in place. The other form of stent uses a method of expanding the stent once it is in place, such as a balloon catheter, Kreamer U.S. Pat. No. 4,740,207 describes one version of the balloon catheter version. In this patent, a semi-rigid tube which has a smaller relaxed diameter which is expanded to a larger operating diameter which Is maintained by a retaining ledge on the Inside of the graft. Concern here, of course, is that the inside located ledge and other retaining means may inadvertently function to cause further blockage of the tube once it is installed. Also. Kreamer states that the tube is held in place by friction between the outer periphery of the graft and the inner periphery of the vessel to prevent displacement of the grant once in place In the vessel. The obvious concern is that the size must be precise or the tube will expand too much or too little, either damaging the vessel or escaping from the location for which it was intended.
Prior art devices represent a foreign body that has no biologically active properties and thus are a factor which contributes in a major way to the eventual restenosis or thrombosis of the vessel. These prior art devices attempt to reduce neointimal hyperplasia passively by adjusting mechanical variables such as lowering the stent profile, coating the stent with carbon, or by making the stent more or less rigid or flexible.
Accordingly, it is an object of the present invention to provide a device and method for deploying stents in blood vessels and other regions of the body without concern for the precise size of the stent being employed or the size of the vessel being treated or repaired.
It is an important object of this invention to produce a stent device and delivery system for the stent which produces rapid endothelialization with the least mount of intimal hyperplasia. While this goal has been stated by others, no effective method or device has been proposed to accomplish that goal.
Another object of this invention is to provide an endoprosthesis device and method for its use in which problems associated with restenosis, thrombosis, infection calcification and/or fibrosis after implantation may be avoided.
Yet another object of the present invention is to provide a device which is effective in administering localized bioactive substances to prevent rejection and side effects from an implanted endoprosthesis device.
Other objects will appear hereinafter.